1. Field of the Invention
The present invention relates generally to a method and device to perform wavelength modulation and more specifically to a method and system for controlling current injection into a Distributed Bragg Reflector (DBR) semiconductor laser to perform wavelength modulation.
2. Background
Lasers have been employed in display technologies for years. In displays such as computer displays, televisions, or the like, colors are generated by the superposition of three primary colors: red, blue and green. As such, within laser-based displays, lasers are employed to provide the primary colors. Each laser can be raster-scanned across the screen or can be stationary and employed to illuminate an image, e.g., a motion picture film or spatial light modulator containing an image. The ability of a laser to provide a beam having excellent brightness characteristics leads to efficient and well-performing lasers within laser-based projectors, when compared to the brightness characteristics of incandescent bulbs used in conventional motion picture theaters.
Laser-based projectors may use single- or multi-wavelength lasers. Single-wavelength semiconductor lasers, such as distributed-Bragg-reflector (DBR) lasers are potential sources for wavelength conversion using a non-linear optical effect. For example, a 1060 nm DBR semiconductor laser tuned to the spectral center of a second-harmonics-generation (SHG) device such as a non-linear crystal may be used to convert the wavelength output by the DBR semiconductor laser to a 530 nm beam. This provides a low-cost, compact and efficient non-linear source of green light.
FIG. 1 schematically illustrates a conventional DBR semiconductor laser 100 and a second harmonic generation (SHG) device 150. The DBR semiconductor laser 100 includes a DBR section or section 110, a phase section or section 120 and a gain section or section 130. The gain section 130, when injected with a continuous wave (CW) current, generates continuous optical power for the laser. The current injected into the DBR section 110 makes large changes to wavelengths output from the laser and the current into the phase section 120 makes small changes to the wavelength of the beam output of the laser. The SHG device 150 receives the beam produced by the semiconductor laser 100. The output intensity of the converted wavelength (green, for example) depends upon alignment of the DBR laser wavelength and the SHG device's spectral center. The beam output from the SHG device 150 is then directed to an output such as display screen.
Generally, for technologies involving video displays, the optical power such as that used to generate the intensity of green light, for example, needs to be modulated at a fundamental frequency of 10 to 100 MHz and with an extinction ratio of approximately 40 dB. The extinction ratio is the ratio of high optical power level to low optical power level. To achieve this combination of high modulation speed and larger extinction ratio remains a daunting task.
One way to obtain a DBR laser 100 and SHG 150 based light source having a fast modulation and a large extinction-ratio is to rapidly modulate the output wavelength of a DBR semiconductor laser 100. As a result, the DBR semiconductor laser beam rapidly scans cross the narrow spectral width of a non-linear SHG device 150 (for example a nonlinear crystal) to produce the necessary intensity modulation. For example, if maximum green power is needed, the DBR wavelength is tuned to the spectral center of the non-linear crystal and, if zero green power is needed a specific time later, the DBR wavelength is tuned outside the spectral width of the non-linear crystal to produce a dark image.
One modulation scheme for use in video is pulse-width wavelength modulation. In this case, the current into the gain section 130 of the DBR laser 100 is maintained at a constant value so that the output intensity of the DBR laser 100 is kept nearly constant, while the current into the DBR section 110 has two possible values: one corresponding to the “on” wavelength that matches to the SHG center wavelength and the other corresponding to the “off” wavelength that is shorter or longer than the SHG center wavelength. The strength or brightness of the green light in each bit period or pixel for the human eyes is determined by the duration in time of current into DBR section 110 corresponding to the “on” wavelength due to the slow response of human eye sensitivity, therefore, the eyes just feel average brightness. Therefore, to modulate the brightness of the green light in each period, the duration (or pulse-width) of current into DBR section 110 corresponding to the “on” wavelength is modulated. This is called “pulse-width modulation”
For a DBR laser 100, the carrier plasma effect is used to dynamically shift the wavelength by injecting current into the DBR section 110. Increasing the current into the DBR section 110 shifts the lasing wavelength to the shorter wavelength end. This is called blue shifting. Reducing the current into the DBR section 110 shifts the laser wavelength to the longer wavelength end. This is called red shifting.
However, the carrier plasma effect described above ignores the possible adverse thermal effect that the injection of current into the laser causes. Injecting current into the DBR section 110 also causes a temperature change in the DBR section 110. For example, if the current corresponding to the “off” wavelength is larger than the current corresponding to the “on” wavelength, then long “off” bits will cause the DBR section temperature to rise, resulting in the lasing wavelength moving toward longer wavelengths, diminishing or completely reversing the required wavelength blue shifting. Alternately, if the current corresponding to the “off” wavelength is smaller than the current corresponding to the “on” wavelength, then long “off” bits will cause the DBR section temperature to fall, resulting in the lasing wavelength moving toward shorter wavelengths, diminishing or completely reversing the required wavelength red shifting. In both instances, at the “on” current that follows, the lasing wavelength is away from the SHG center wavelength, resulting in an undesirable low conversion efficiency.
In a video display application, the DBR section 110 temperature at a specific time depends on the history of previous bits. The heating caused by current injection of the “on” wavelength and “off” wavelength during previous bits affects the wavelength of the current bit. Therefore, at any particular time, the thermal effect can cause the laser wavelength to drift away from the center wavelength of an SHG 150 even if the “on” wavelength is applied, reducing the conversion efficiency of the SHG 150. FIG. 2 shows this effect, known as a thermally-induced patterning effect. In FIG. 2, the temperature of the DBR section 110 increases when a constant “on” current is applied, resulting in a longer actual wavelength and less intense SHG output over time. Likewise, the temperature of the DBR section 110 decreases when a lower constant “off” current is applied, resulting in a shorter actual wavelength over time.